1. Field of the Invention
The present invention is directed to the inhibition of polymerization and polymer growth of ethylenically unsaturated monomers by means of the addition thereto of hydrogen donors and/or electron acceptors, either alone or in combination with at least one stable nitroxide free radical compound.
2. Description of Related Art
Many ethylenically unsaturated monomers undesirably polymerize at various stages of their manufacture, processing, handling, storage, and use. Polymerization, such as thermal polymerization, during their purification results in the loss of the monomer, i.e., a lower yield, and an increase in the viscosity of any tars that may be produced. The processing and handling of the higher viscosity tars then requires higher temperature and work (energy cost) to remove residual monomer.
Polymerization can also result in equipment fouling, especially in the case of production of acrylic monomers. Such polymerization causes loss in production efficiency owing to the deposition of polymer in or on the equipment being used. These deposits must be removed from time to time, leading to additional loss in production of the monomer.
A wide variety of compounds has been proposed and used for inhibiting uncontrolled and undesired polymerization of ethylenically unsaturated monomers. However, many of these compounds have not been fully satisfactory.
There are several mechanisms by which polymerization inhibitors work. One mode of action for polymerization inhibitors is for the inhibiting species to combine with the propagating polymer chain such that the polymerization of that polymer chain stops, i.e., a termination reaction. If such an inhibitor-terminated polymer chain is capable of participating in a dynamic equilibrium between a dormant species (the inhibitor-terminated chain) and an active polymer chain, it would be considered a “living” or quasiliving polymer. For example, Ivan, Macromol. Symp. 88:201-215 (1994) describes quasiliving polymerization as a polymerization in which “ . . . only a portion of chain ends are active (propagating) and these are in equilibria with inactive (dormant, nonpropagating) chains . . . ” Shigemoto et al., Macromol. Rapid Commun. 17:347-351 (1996) state, “Well-defined polymers can be prepared by controlled/“living” radical polymerization in the presence of relatively stable radicals. These systems employ the principle of dynamic equilibration between dormant species and growing radicals via reversible homolytic cleavage of a covalent bond in dormant species.” Further, Greszta et al., Macromolecules 29:7661-7670 (1996) state, “The reversible homolytic cleavage of dormant species can be accomplished by either thermal, photochemical, or catalytic activation. The most successful approaches are as follows: homolytic cleavage of alkoxyamines and dithiocarbamates, use of various organometallic species, and catalyzed atom transfer radical polymerization.” Such a “living” polymer is capable of increasing in molecular weight (growing) through its reaction with additional monomer units of the same or different types of polymerizable monomers.
The method by which this “living” polymer grows is termed the “living” polymerization mechanism, and is depicted below.M−Inh----->M*+*Inh  (1)M*+*Inh---->M−Inh  (2)M*+M′---->M−M′*  (3)M−M′*+*Inh---->M−M′−Inh  (4)Reactions (1) and (2) depict the dynamic equilibrium, with (2) being the termination reaction. Reaction (3) depicts growth of the polymer chain. Reaction (4) depicts re-termination of the growing polymer chain with the inhibiting species. The amount of growth over any period of time is dependent on the relative rate at which (2) occurs versus (3), as long as (1) occurs to some extent. The faster (2) is relative to (3), the more time is needed for significant growth of the polymer. Under the conditions in which inhibitors are normally used, the concentration of the inhibiting species should be sufficiently high to cause reaction (2) to be much faster than reaction (3), otherwise it would not be an effective inhibiting system for commercial use. However, we have realized that even at an effective inhibiting amount of the inhibitor, growth can still occur, given sufficient time and temperature.
There are at least two scenarios in which “living” polymer can remain in a monomer purification train for an excessive amount of time.
First, the use of recycle can significantly increase the amount of time that the “living” polymer can remain in the purification train. To recycle unused inhibitor that is left in the purification stream after removal of the monomer, a portion of the residual stream is added to a feed stream earlier in the purification train. This residual stream typically contains inhibitor, small amounts of monomer, impurities in the monomer stream that have been concentrated by the purification process, and polymer formed during the production and purification process. Recycling this polymer will allow it time to grow if it is “living” polymer and the conditions of the purification train allow the “living” polymerization mechanism to occur. If this polymer grows via the “living” polymerization mechanism, excessive polymerization would cause loss in product yield, increased waste residues from the process, and potential plugging of equipment due to excessively high molecular weight polymer in the purification stream.
Second, occasionally, conditions in the plant/purification process can result in the formation of polymer within the purification train that is not dissolved by the monomer stream. If this polymer is caught in a dead space, or if it attaches to the metal on the inside of the equipment, it will not be washed out of the system. Thus, the polymer will remain within the system indefinitely (potentially for two or more years). If this polymer grows via the “living” polymerization mechanism, it could coat the inside of the equipment, causing inefficient separation of the monomer stream components and/or insufficient heating of the stream to enable purification. Such a situation would cause loss in product yield and could potentially cause an unscheduled shut-down of the plant in order to clean out the undissolved polymer in the equipment. Such a shut-down results in loss of monomer production and additional expense to clean out and dispose of the undissolved polymer.
It is significant that there has been no indication that previously used inhibitors would lead to the formation of “living” polymer when used as polymerization inhibitors. However, a newly utilized class of inhibitors, the stable nitroxyl radicals, is known to allow this “living” polymerization mechanism to occur. These nitroxyl radicals are highly efficient polymerization inhibitors under normal use, providing better performance than most other inhibitors on the market, but their incapacity to prevent “living” polymerization has hindered their full utilization. Accordingly, there is a need for compositions that can be used in a purification train, preferably in combination with nitroxyl radicals, to prevent polymer growth that occurs via a “living” polymerization mechanism.
Nitroxyl radicals are known to facilitate polymerization via a “living” free radical process to give polymers of narrow polydispersity.
Georges et al., Macromolecules 26(11):2987-2988 (1993) synthesized narrow molecular weight resins by a free-radical polymerization process with polydispersities comparable to those that can be obtained by anionic polymerization processes and below the theoretical limiting polydispersity of 1.5 for a conventional free-radical polymerization process. The process comprised heating a mixture of monomer(s), free-radical initiator, and a stable free radical, e.g., 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO).
Hawker et al., Macromolecules 29(16):5245-5254 (1996) prepared and evaluated a variety of initiating systems for the preparation of macromolecules by nitroxide-mediated “living” free radical procedures. The systems were divided into two classes, unimolecular initiators in which alkylated TEMPO derivatives dissociate to provide both the initiating radical and the stable radical, and bimolecular systems in which a traditional free radical initiator, such as BPO or AIBN, is used in conjunction with TEMPO. For the unimolecular initiators it was found that an α-methyl group is essential for “living” character, while a variety of substituents could be placed on the phenyl ring or the β-carbon atom without affecting the efficiency of the unimolecular initiator. It was found that the rate of polymerization is approximately the same for both the unimolecular and corresponding bimolecular systems; however, the unimolecular initiators afforded better control over molecular weight and polydispersity.
The inventors are unaware of any art on the use of compounds to prevent polymer growth that occurs via a “living” polymerization mechanism since this growth phenomenon is not known to have previously been observed. Hindered nitroxyl compounds are known to be very active inhibitors of free radical polymerizations of unsaturated monomers such as styrene, acrylic acid, methacrylic acid, and the like. Nitrophenols, nitrosophenols, phenylenediamines (PDA's), hydroxylamines, quinones and hydroquinones are also known to have a similar capacity.
U.S. Pat. No. 2,304,728 discloses that a vinyl aromatic compound may effectively be stabilized against polymerization by dissolving therein a monohydric halonitrophenol having the general formula:
wherein one X represents a halogen and the other X represents a member of the group consisting of hydrogen and halogen and nitro substituents.
U.S. Pat. No. 3,163,677 discloses a process for the preparation of N,N,O-trisubstituted hydroxylamines and N,N-disubstituted nitroxides of the formulae:
wherein R1, R2, and R3 are each an alkyl radical having 1 to 15 carbon atoms. (As used herein, the designation N—O* denotes a stable free radical wherein the asterisk is an unpaired electron.) The N,N,O-trisubstituted hydroxylamines can be used to make the N,N-disubstituted nitroxides, which are stable free radicals and are said to be useful as polymerization inhibitors.
U.S. Pat. No. 3,334,103 discloses that nitroxides can be prepared from the corresponding heterocyclic amine wherein the nitrogen atom of the nitroxide group is attached to other than a tertiary carbon of an aliphatic group (i.e., the nitrogen atom forms a part of a heterocyclic nucleus). These nitroxides are said to have useful properties similar to those described for the N,N-disubstituted nitroxides of U.S. Pat. No. 3,163,677.
U.S. Pat. No. 3,372,182 discloses that a great variety of N,N-disubstituted, stable, free radical nitroxides not otherwise readily available can be prepared by a simple and convenient process that comprises pyrolyzing in an inert reaction medium virtually any hydroxylamine that is susceptible to cleavage of the O—C bond, e.g., tri-t-butylhydroxylamine.
U.S. Pat. No. 3,422,144 discloses stable, free radical nitroxides of the formula:
wherein R is selected from the group consisting of tertiary alkyl, aryl, alkaryl, haloaryl, carboxyaryl, alkoxyaryl, alkylthioaryl, pyridyl, and dialkylaminoaryl, and R′ is tertiary alkyl. These nitroxides are said to be useful as traps for reactive free radicals both in the counting of free radicals and for inhibiting oxidation and free radical polymerization.
U.S. Pat. No. 3,494,930 discloses free radicals of the nitroxide type for use as initiators of free radical reactions, collectors of free radicals, polymerization inhibitors or antioxidants. They are constituted by nitrogenous bicyclic compounds in which one of the bridges comprises solely the nitroxide radical group and, in particular, by aza-9-bicyclo(3,3,1) nonanone-3-oxyl-9, and by aza-9-bicyclo(3,3,1) nonane oxyl-9.
U.S. Pat. No. 3,873,564 discloses compounds and a method for assaying enzymes by adding to a medium containing an enzyme a stable free radical compound having a stable free radical functionality which, when subjected to an enzyme-catalyzed reaction, changes the environment of the free radical functionality. By following the change in the electron spin resonance spectrum as affected by the change in environment, the type of enzyme and the activity of the enzyme can be determined. The compounds found useful are normally stable nitroxide radicals with an enzyme labile functionality. Other compounds include two cyclic nitroxide containing rings joined by a chain having an enzyme labile functionality.
U.S. Pat. No. 3,966,711 teaches that 2,2,7,7-tetraalkyl- and 2,7-dispiroalkylene-5-oxo-1,4-diazacycloheptanes substituted in the 4-position by mono- or tetravalent radicals are powerful light-stabilizers for organic polymers. They are said to possess higher compatibility than their 4-unsubstituted homologues, from which they can be synthesized by reactions known for N-alkylation. Preferred substituents in the 4-position are alkyl, alkylene, alkenyl, aralkyl, and esteralkyl groups. The 1-nitroxyls derived from the imidazolidines by oxidation with hydrogen peroxide or percarboxylic acids are also said to be good light stabilizers.
U.S. Pat. No. 4,105,506 discloses a process for the distillation of readily polymerizable vinyl aromatic compounds and a polymerization inhibitor therefor. The process comprises subjecting a vinyl aromatic compound to elevated temperatures in a distillation system in the presence of a polymerization inhibitor comprising 2,6-dinitro-p-cresol.
U.S. Pat. Nos. 4,252,615 and 4,469,558 disclose a process for the distillation of readily polymerizable vinyl aromatic compounds and a polymerization inhibitor therefor. The process comprises subjecting a vinyl aromatic compound to elevated temperatures in a distillation system in the presence of a polymerization inhibitor comprising 2,6-dinitro-p-cresol. Also disclosed is a distillation method and apparatus for use with this inhibitor.
U.S. Pat. No. 4,434,307 discloses the stabilization of vinyl aromatic compounds against undesired polymerization by adding to the vinyl aromatic compounds small amounts of at least one N,N-diarylhydroxylamine and at least one mono- or ditertiary alkyl catechol and/or at least one mono- or ditertiary alkylhydroquinone.
U.S. Pat. No. 4,439,278 discloses an improvement in methods for preparing and processing ethylenically unsaturated aromatic monomer. The improvement comprises employing 3,5-dinitrosalicylic acid or a derivative or isomer thereof as a process inhibitor. The process inhibitor is present in a concentration of about 50 to 3000 ppm, preferably about 250 to 2,000 ppm, and most preferably about 500 to 1,000 ppm.
U.S. Pat. No. 4,466,904 discloses a compound and a process for utilizing the compound to prevent the polymerization of vinyl aromatic compounds, such as styrene, during heating. The compound includes effective amounts of phenothiazine, 4-tert-butylcatechol and 2,6-dinitro-p-cresol respectively, as a polymerization inhibitor system in the presence of oxygen resulting in a less viscous polymer tar and in the effective inhibition of polymerization to temperatures as high as 150° C.
U.S. Pat. Nos. 4,466,905 and 4,468,343 disclose a compound and a process for utilizing the compound to prevent the polymerization of vinyl aromatic compounds, such as styrene, during heating. The composition includes effective amounts of 2,6-dinitro-p-cresol and either a phenylenediamine or 4-tert-butylcatechol respectively, to act as a polymerization co-inhibitor system in the presence of oxygen.
U.S. Pat. No. 4,480,116 discloses an improvement in methods for preparing and processing readily polymerizable acrylate monomers. The improvement comprises employing phenyl-para-benzoquinone, 2,5-di-phenyl-para-benzoquinone, and mixtures thereof as process inhibitors. The process inhibitors are present in a concentration of about 50 to 3000 ppm, preferably about 250 to 2000 ppm, and most preferably about 500 ppm.
U.S. Pat. No. 4,558,169 discloses a process for preparing vinyltoluene comprising passing ethyltoluene through a dehydrogenation zone to form vaporous crude vinyltoluene, adding from about 50 to about 100 parts per million by weight of a polymerization inhibitor such as a nitrated phenol to the vaporous crude vinyltoluene at a temperature between about 200° and about 300° C., condensing the vaporous crude vinyltoluene, maintaining the pH of the aqueous phase of the condensed crude vinyltoluene at a value between about 5.5 and about 6.5 sufficient to maintain the inhibitor in the organic phase of the condensed crude vinyltoluene, adding a second portion of polymerization inhibitor to the condensed crude vinyltoluene until the inhibitor concentration totals about 500 parts per million by weight relative to the vinyltoluene content of the crude vinyltoluene, filtering the condensed crude vinyltoluene to remove seed polymer, and distilling the condensed crude vinyltoluene to recover substantially pure vinyltoluene; and apparatus for carrying out said method.
U.S. Pat. No. 4,665,185 discloses a process for the efficient preparation of nitroxyls of sterically hindered amines by the oxidation of the amine using a hydroperoxide in the presence of a small amount of a metal ion catalyst, at moderate temperature for a short period of time, to give the nitroxyl in high yield and purity.
U.S. Pat. No. 4,692,544 discloses certain substituted diaryl amines that are used to inhibit the polymerization of ethylenically unsaturated monomers; for example, unsaturated carboxylic acids and derivatives thereof.
U.S. Pat. No. 4,720,566 discloses compositions and methods of inhibiting acrylonitrile polymerization, particularly in quench columns of systems producing acrylonitrile, comprising adding to the acrylonitrile an effective amount for the purpose of (a) a hydroxylamine having the formula
wherein R and R′ are the same or different and are hydrogen, alkyl, aryl, alkaryl or aralkyl groups, and (b) a para-phenylenediamine or derivative thereof having at least one N—H group. Preferably the phenylenediamine is a para-phenylenediamine having the formula
wherein R1, R2, R3 and R4 are the same or different and are hydrogen, alkyl, aryl, alkaryl, or aralkyl groups with the proviso that at least one of R1, R2, R3 or R4 is hydrogen.
U.S. Pat. No. 4,774,374 discloses a vinyl aromatic composition stabilized against polymerization comprising (a) a vinyl aromatic compound and (b) an effective amount of a stabilizer system in which the active ingredient consists essentially of an oxygenated species formed by the reaction of oxygen and an N-aryl-N′-alkyl-p-phenylenediamine.
U.S. Pat. No. 4,797,504 discloses compositions and methods of inhibiting acrylate monomer polymerization at elevated temperatures comprising adding to the acrylate monomer an effective amount for the purpose of (a) a hydroxylamine having the formula
wherein R and R′ are the same or different and are hydrogen, alkyl, aryl, alkaryl or aralkyl groups, and (b) a para-phenylenediamine or derivative thereof having at least one N—H group. Preferably the phenylenediamine is a para-phenylenediamine having the formula
wherein R1, R2, R3 and R4 are the same or different and are hydrogen, alkyl, aryl, alkaryl, or aralkyl groups with the proviso that at least one of R1, R2, R3 or R4 is hydrogen.
U.S. Pat. No. 4,912,247 discloses a composition and method of use for inhibiting the polymerization of acrylate esters during elevated temperature processing and during storage and handling thereafter. It comprises the combination of a Mannich reaction product, which is prepared from a substituted phenol, an aldehyde and ethylenediamine, and either phenylenediamine or derivatives thereof and/or phenothiazine or derivatives thereof.
U.S. Pat. No. 4,929,778 discloses methods and compositions for inhibiting the polymerization of styrene monomer during elevated temperature processing thereof or during storage or shipment of styrene containing product. The compositions comprise a combination of (a) a phenylenediamine compound having at least one N—H bond and (b) a hindered phenol compound. The methods comprise adding from 1-10,000 ppm of the combination to the styrene medium, per one million parts of styrene.
U.S. Pat. No. 5,128,022 discloses methods and compositions for inhibiting the formation of polymers in petroleum or petrochemical processes that subsequently foul heat transfer surfaces. The compositions comprise a combination of N-Phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine (PDA) and an organic acid. The methods comprise adding from 1 to 2500 ppm PDA and 1 to 3500 ppm organic acid to the system experiencing the fouling problem.
U.S. Pat. No. 5,254,760 teaches that the polymerization of a vinyl aromatic compound, such as styrene, is very effectively inhibited during distillation or purification by the presence of at least one stable nitroxyl compound together with at least one aromatic nitro compound.
U.S. Pat. No. 5,446,220 discloses methods for inhibiting the polymerization of vinyl aromatic monomers in oxygen-free processing systems. These methods comprise adding from 1 to about 10,000 parts per million parts monomer of a combination of a dinitrophenol compound, a hydroxylamine compound and a phenylenediamine compound. Preferably, 2-sec-butyl-4,6-dinitrophenol or 4,6-dinitro-o-cresol are used in combination with bis-(hydroxypropyl)hydroxylamine and N,N′-di-sec-butyl-p-phenylenediamine.
U.S. Pat. Nos. 5,545,782 and 5,545,786 disclose that nitroxyl inhibitors in combination with some oxygen reduce the premature polymerization of vinyl aromatic monomers during the manufacturing processes for such monomers. Even small quantities of air used in combination with the nitroxyl inhibitors are said to result in vastly prolonged inhibition times for the monomers.
European Patent Application 0 178 168 A2 discloses a method for inhibiting the polymerization of an α,β-ethylenically unsaturated monocarboxylic acid during its recovery by distillation by using a nitroxide free radical.
European Patent Application 0 325 059 A2 discloses stabilizing vinyl aromatic compounds against polymerization by the addition of an effective amount of a polymerization inhibition composition comprising (a) a phenothiazine compound; and (b) an aryl-substituted phenylenediamine compound.
European Patent Application 0 398 633 A1 discloses a method of inhibiting acid monomer polymerization comprising adding to the monomer (a) a manganese source compound and (b) a phenylenediamine compound having at least one N—H bond therein.
European Patent Application 0 594 341 A1 discloses methods and compositions for inhibiting the polymerization of vinyl aromatic monomers under distillation conditions. The compositions comprise a combination of a phenylenediamine compound and a hydroxylamine compound.
European Patent Application 0 765 856 A1 discloses a stabilized acrylic acid composition in which the polymerization of the acrylic acid is inhibited during the distillation process for purifying or separating the acrylic acid as well as during transport and storage. The compositions comprise three components: (a) acrylic acid, (b) a stable nitroxyl radical, and (c) a dihetero-substituted benzene compound having at least one transferable hydrogen (e.g., a quinone derivative such as the monomethyl ether of hydroquinone (MEHQ)). During the distillation process, transport, and storage, components (b) and (c) are present in a polymerization-inhibiting amount. During the distillation process, oxygen (d) is preferably added with components (b) and (c). According to the specification, examples of suitable nitroxide free radical compounds include di-t-butylnitroxide; di-t-amylnitroxide; 2,2,6,6-tetramethyl-piperidinyloxy; 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy; 4-oxo-2,2,6,6-tetramethyl-piperidinyloxy; 4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy; 4-amino-2,2,6,6-tetramethyl-piperidinyloxy; 4-ethanoyloxy-2,2,6,6-tetramethyl-piperidinyloxy; 2,2,5,5-tetramethylpyrrolidinyloxy; 3-amino-2,2,5,5-tetramethylpyrrolidinyloxy; 2,2,5,5-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy; 2,2,5,5-tetramethyl-1-oxa-3-pyrrolinyl-1-oxy-3-carboxylic acid; and 2,2,3,3,5,5,6,6-octamethyl-1,4-diazacyclohexyl-1,4-dioxy.
WO 97/46504 concerns substance mixtures containing: (A) monomers containing vinyl groups; and (B) an active amount of a mixture which inhibits premature polymerization of the monomers containing vinyl groups during their purification or distillation and contains: (i) between 0.05 and 4.5 wt %, relative to the total mixture (B), of at least one N-oxyl compound of a secondary amine which has no hydrogen atom at the sa-C atoms; and (ii) between 99.95 and 95.5 wt % relative to the total mixture (B), of at least one nitro compound. The publication also discloses a process for inhibiting the premature polymerization of monomers, and the use of mixture (B) for inhibiting the premature polymerization of monomers.
WO 98/02403 relates to inhibiting the polymerization of vinyl aromatic compounds by using a mixture of a phenol and a hydroxylamine. It is said that the process is useful in ethylbenzene dehydrogenation effluent condenser systems and styrene-water separator vent gas compressor systems and that it effectively inhibits polymerization of monomers, preventing the formation of a polymer coating on condenser and compressor equipment, thus reducing the necessity for cleaning of equipment surfaces.
WO 98/14416 discloses that the polymerization of vinyl aromatic monomers such as styrene is inhibited by the addition of a composition of a stable hindered nitroxyl radical and an oxime compound.
WO 98/25872 concerns substance mixtures containing: (A) compounds containing vinyl groups; (B) an active amount of a mixture which inhibits premature polymerization of the compounds containing vinyl groups and contains: (i) at least one N-oxyl compound of a secondary amine which does not carry any hydrogen atoms on the α-carbon atoms; and (ii) at least one iron compound; (C) optionally nitro compounds; and (D) optionally co-stabilizers. The publication also discloses a process for inhibiting the premature polymerization of compounds (A) containing vinyl groups, and the use of (B) optionally mixed with nitro compounds (C) and/or co-stabilizers (D) for inhibiting the premature polymerization of radically polymerizable compounds and stabilizing organic materials against the harmful effect of radicals.
U.K. Patent Number 1,127,127 discloses that acrylic acid can be stabilized against polymerization by the addition thereto of a nitroxide having the essential skeletal structure:
wherein R1, R2, R3, and R4 are alkyl groups and no hydrogen is bound to the remaining valencies on the carbon atoms bound to the nitrogen. The two remaining valencies that are not satisfied by R1 to R4 or nitrogen can also form part of a ring (e.g., 2,2,6,6 tetramethyl-4-hydroxy-piperidine-1-oxyl).
CS-260755 B1 is directed to the preparation of 4-substituted-2,2,6,6-tetramethylpiperidine nitroxyls as olefin stabilizers.
SU-334845 A1 is directed to the inhibition of the radical polymerization of oligoester acrylates using iminoxyl radical inhibitors of a given formula.
SU-478838 is directed to the inhibition of the radical polymerization of oligoester acrylates and the prevention of oligomeric peroxides using a binary polymerization inhibitor comprising quinone.
FR 2,761,060 relates to the prevention of premature polymerization of styrene during its production by dehydrogenation of ethylbenzene by injecting into the process effluent a radical inhibitor based on an oxyl-tetramethylpiperidine derivative.
The foregoing are incorporated herein by reference in their entirety.